Conventionally, there is a configuration called multihoming that facilitates connections with a plurality of Internet service providers (ISPs). The state wherein a site has a multihoming configuration is especially called site multihoming. Hereinafter, a site that has a multihoming configuration is called a multihomed site, and a subnet (a subnetwork) belonging to a multihomed site is called a site multihoming network.
Site multihoming will now be described while referring to FIG. 16. FIG. 16 is a diagram showing an example network configuration for explaining site multihoming for the conventional art.
In FIG. 16, a site 1 is shown that establishes a connection with a plurality of ISPs (an ISP1 and an ISP2) and that maintains, via these ISPs, an access to the Internet 1601, which is an IP network. This site 1 also includes a subnet A and a subnet B. Therefore, the site 1 is a multihomed site, and the subnets A and B are site multihoming networks.
Site multihoming is a technique used for multiplexing a connection path for the Internet 1601, and the employment of site multihoming produces an effect such as an improvement in failure proofing for accessing the Internet 1601 using a communication node 1602 in site 1. For example, the communication node 1602 in site 1 is so designed that it enables the accessing of the Internet 1601 via two ISPs, i.e., ISP1 and ISP2, or in communicating with a correspondent node (CN) via the Internet 1601.
Since prefixes (network prefixes) are respectively transmitted by ISP1 and ISP2 to site 1 shown in FIG. 16, these prefixes can be employed in the subnets that belong to site 1. Therefore, for example, the communication node 1602 that is connected to subnet A can generate a plurality of addresses formed using the prefixes for both ISP1 and ISP2.
In order to obtain a multihoming effect, the communication node 1602 must switch addresses, as needed, to be used for communication. For a case involving a transmission packet, the multihoming effect is obtained by employing a source address to determine which ISP is to be passed through, and for a case involving a reception packet, the multihoming effect is obtained by employing a destination address to determine which ISP is to be passed through.
Furthermore, as a method whereby a communication node that has been set in a multihomed state by the site multihoming employs a plurality of addresses to communicate with a correspondent node, the IETF SHIM6 Working Group has proposed a method whereby a plurality of addresses are managed within a network layer, and for an upper layer, addresses are mapped using a single identifier, so that the presence of a plurality of addresses is hidden (see, for example, non-patent document 1 below).
Further, reasons that the communication node switches addresses can, as described in non-patent document 2 below, be cases, for example, wherein various switching reasons have occurred in accordance with communication statuses, such as a case wherein a explicit notification, such as a disconnection notification, etc., has occurred because an Ack (Acknowledgement) message was not received from a TCP (Transfer Connection Protocol) layer, a case wherein a recovery is to be performed upon the occurrence of a failure due to an ISP that is currently being employed, a case wherein a load imposed on an ISP is to be dispersed, and a case wherein congestion is to be controlled.
Further, various other factors are also possible, such as a case wherein an ISP to be employed is changed in accordance with an MN (Mobile Node), a CN (Correspondent node), or an HA (Home Agent) that manages the address of the MN and the contents of preference information for a home network (i.e., a case wherein the occurrence of a reason for switching is other than a reason related to a communication status caused by the communication node side or a network side that includes the HA). Furthermore, as a method for selecting a new address to be employed after these reasons for switching have occurred, a static selection method described in non-patent document 3 below, or a method, as described in non-patent document 2, for example, for employing the results obtained by dynamically checking the probability that the address has been reached, can be employed.
When a communication node is to transmit a packet to a correspondent node by switching source addresses, the communication node must notify the correspondent node in advance of a plurality of addresses to be employed for this switching, and a message for transmitting this information is also referred to in non-patent document 1. It should be noted that, as a method for notifying the correspondent node of a plurality of addresses of the communication node that is a source, not only a method proposed in non-patent document 1, but in addition, for example, there is a method, described in non-patent document 4, for enclosing the addresses in a message related to a different protocol, such as a binding update message for a mobile IP, that can also be employed. Thus, even when receiving packets for which different source addresses are designated, the correspondent node can determine that they were transmitted using the same communication node.
A mobile IP described in non-patent document 4 will be briefly described. For an MN that is a communication node in the mobile IP, at least one HoA (Home Address) is assigned that is related to its own home network. In a case wherein this MN has been moved to another subnetwork (a foreign network), the MN obtains at least one CoA (Care-of Address) in the subnetwork to which the MN was moved, and notifies the HA in the home network of information (binding information) indicating that a correlation exists between the obtained CoA and the assigned HoA in the home network. Therefore, since the HA receives, as a proxy, a packet transmitted to the HoA of the MN, and transfers the packet addressed to the CoA, the MN can receive the packet addressed to the HoA, even in a case wherein the MN is present in the foreign network.
In addition, in a case wherein an MN that was moved to a foreign network transmits a packet, the MN generates an encapsulated packet (an outer packet) by encapsulating a packet addressed to the HA, and transmits the encapsulated packet. FIG. 17 is a diagram showing an example encapsulated packet to be generated, according to the conventional art, in a case wherein an MN transmits a packet to a CN. As shown in FIG. 17, the address of the HA is set as a destination address in the header (the external header) of an encapsulated packet, and the CoA of the MN is set as a source address. On the other hand, since the inner packet is the packet for substance to be delivered to a CN, the address of the CN is set as a destination address, and the HoA of the MN is set as a source address.
When the HA designated as the destination address in the external header receives the encapsulated packet transmitted by the MN, the HA extracts the inner packet, by decapsulating the received packet, and transfers the extracted inner packet. The CN receives this as a normal packet transmitted by the MN. Through this processing, the MN in the foreign network can still transmit, to the CN, a packet for which the HoA of the MN is designated as a source address.
Further, in a case wherein the home network of an MN is a site multihoming network, since a plurality of prefixes are valid on this home network, the MN can employ a plurality of home addresses. And when the MN is not moved (i.e., the MN is connected to its own home network), the connected network (the home network) is a site multihoming network, so that an address (which may be called an appropriate address) that it is desirable that the MN employs can be determined, while taking into account address information, which is exchanged with the correspondent node, and a communication status.
Non-patent Document 1: Erik Nordmark, Marcelo Bagnulo, “Multihoming L3 Shim Approach”, draft-ietf-multi6-13shim-00.txt, 10 Jan. 2005.
Non-patent Document 2: J. Arkko, “Failure Detection and Locator Selection in Multi6”, draft-ietf-multi6-failure-detection-00.txt, January 2005.
Non-patent Document 3: R. Draves, “Default Address Selection for Internet Protocol version 6 (IPv6)”, RFC3484, February 2003.
Non-patent Document 4: Johnson, D. B., Perkins, C. E., and Arkko, J., “Mobility Support in IPv6”, RFC3775, June 2004.
However, in a case wherein, when an MN, which belongs to a home network that serves as a site multihoming network provided by a plurality of ISPs and which holds a plurality of HoAs, has moved to a foreign network and transmits a packet to a correspondent node, a phenomenon regarded as a change deterioration, like the stopping of a service due to a failure, a congested state change or a roaming state change between the ISPs has occurred in one of the plurality of ISPs, a packet may not be delivered, or the transmission or reception of a packet may be delayed when the HoA, formed of the prefix of the ISP in the deteriorated state, is selected as an address to be used.
In this case, since the HA of the MN is present in the same home network and a plurality of addresses can be obtained, when the MN that has moved from the home network transmits a encapsulated packet to the HA, an ISP to be passed by the HA is determined in accordance with a destination address that has been designated. Therefore, the MN and HA exchange address information using a multihoming protocol, the purpose of which is that the site multihoming effects provided by the HA are applied for the MN-HA communication, and that the information exchanged by the MN and HA does not include information concerning site multihoming for the home network of the MN.
That is, although an inappropriate ISP or a desirable ISP to be employed for packet transmission is present among a plurality of ISPs that provide, for the home network, the connection to the Internet, the MN is not connected to the home network, so that information related to the status at the home network can not be obtained. For of the above described reason, a problem has arisen, in that since the MN can not determine an HoA that should be designated as a source address in the internal header of an encapsulated packet, the inner packet is transmitted via an inappropriate ISP.
Moreover, since a packet to be transferred by the HA corresponds to the inner packet of the encapsulated packet transmitted by the MN, an ISP through which the transferred packet is passed is determined in accordance with a source address that is designated by the MN based on a specific determination reference. At this time, since generally there are multiple MNs managed by one HA, the HA manages, at the same time, a plurality of MNs that are moving to various other networks. Since the setting of a source address for an inner packet basically depends on determinations made by the individual MNs, it can be said that an HA that manages a plurality of MNs transfers not only a packet that is passed via a specific ISP, but also a packet that is passed via another ISP, without identifying them for each other. Therefore, as a result of a normal packet transfer process, the HA can sequentially obtain the affect had by each ISP on a packet that has been transferred.
On the other hand, while the MN can obtain the status of the ISP that is currently employed as the source address for the inner packet, the MN can not acquire the status of an ISP whose use has been temporarily halted, or the status of an ISP whose presence is not known. Furthermore, in a case wherein the MN includes a plurality of interfaces, which correspond to the respective ISPs, in order to confirm the status of an ISP that has not been employed as the source address for an inner packet, the MN must render a corresponding interface active, transmitting a signal each time, so that power consumption and the volume of traffic are increased. Further, the same situation is applied for a case wherein the individual interfaces included in the MN belong to different home networks provided by different ISPs, and in order to obtain the status of a specific ISP (a home network), a corresponding interface must be rendered active and a signal must be transmitted.